Magneto-resistive effects include a number of different physical phenomena, all of which having in common that an electrical resistance of a resistive element is alterable by the behavior of a magnetic field penetrating the resistive element. Technologies utilizing magneto-resistive effects are sometimes referred to as “XMR technologies”, whereby the X indicates that a multitude of effects may be addressed here.
One example is the Anisotropic Magneto-Resistive (AMR) effect, which is based on the fact that in a (nanometer-) thin conductive layer the electrical resistance may be changed by altering an angle between an external magnetic field and a direction of an electric current within the layer plane. The effect may be explained by a distortion of atomic orbitals due to spin orientation in a magnetic field and hence different scattering cross sections of conducting electrons.
Another example is the Giant Magneto-Resistive (GMR) effect, which may occur in a layer stack of layers comprising at least two ferromagnetic layers separated by a non-ferromagnetic layer. If both ferromagnetic layers are magnetized the electrical resistance of the layer stack may be altered by changing the angle between the magnetization directions of the ferromagnetic layers, whereby the effect results from electron scattering depending on spin orientation of the electrons. The different magnetization directions may be achieved by pinning one ferromagnetic layer (pinned layer) to a reference magnetization, whereas the magnetization of the other ferromagnetic layer (free layer) may depend on an external magnetic field.
Yet another example is the Tunnel Magneto-Resistive (TMR) effect, which may occur in a layer stack of (nanometer-) thin layers comprising at least two ferromagnetic layers separated by an electrically isolating layer. If both ferromagnetic layers are magnetized the electrical resistance of the layer stack may be altered by changing the angle between the magnetization directions of the ferromagnetic layers, whereby the effect results from tunneling probability depending on the orientations of electron spin and of the magnetic fields. Again, the different magnetization directions may be achieved by pinning one ferromagnetic layer (pinned layer) to a reference magnetization, whereas the magnetization of the other ferromagnetic layer (free layer) may depend on an external magnetic field.
XMR effects may be applied in a variety of field based sensors, for example for measuring revolution, angles, etc. In some applications, especially in applications relevant to safety, it is required that these sensors operate reliably and at a high level of accuracy. Conventional solutions comprise redundancy concepts featuring two independently manufactured sensors, which are expensive in terms of production effort and cost. Conventional solutions further comprise safety algorithms that have only limited capability, resulting in unrecognized errors. As a result, the price for a XMR sensor significantly increases with its functional safety features.
It is hence desirable to improve a compromise between reliability, accuracy, production effort and cost of XMR sensors.